Add diagnostics functions: cpu_mem_plot, error_count, tf_delay

[Kotakku]

baglab.diagnostics — additional functions

Function	Purpose	Priority
cpu_mem_plot(diagnostic_topic)	CPU/memory usage over time (DiagnosticArray)	Medium
error_count(diagnostic_agg)	Error count from DiagnosticArray	Medium
tf_delay(bag, parent, child)	TF delay and dropout check	Medium